The present invention relates generally to lighting devices for driving a semiconductor light-emitting element such as a light-emitting diode (LED), and associated illumination fixtures. More particularly, the present invention relates to LED lighting devices with an output impedance element and associated control circuitry for stabilizing an optical output.
Lighting devices for driving a semiconductor light-emitting element are known in the art which can control an optical (lighting) output across a wide range, from a very weak optical output to an optical output of a rated current. One example includes a circuit configuration with a current divider connected in parallel with the semiconductor light-emitting element and diverting a driving current flowing to the semiconductor light-emitting element. A resistor, a current regulation diode or a thermistor may be used as specific examples of the current divider.
A typical application of such a technique, such as may be used in an inspection light source for a solid-state image sensing element, includes an LED driver circuit for sending a relatively small current to an LED with high accuracy. The driver circuit may include a D/A converter and an analog driver. Such an LED driver circuit is relatively expensive and inefficient, making it unsuitable for many illumination fixtures as would be used in homes and offices. Further, power losses due to the current divider are simply disregarded.
In another example, a switching power supply device as known in the art for controlling a semiconductor light-emitting element across a wide range of lighting outputs performs constant current control for outputs near a rated current (high end of the lighting range) so as to match an output current of a switching power supply with a target current value, and performs constant voltage control for outputs at the low end of the lighting range so as to match an output voltage of the switching power supply with a target voltage value.
According to this technique, as compared to the first technique described above, power loss is decreased due to the switching power supply device. However, because the technique requires both a feedback control system for constant current control used in the vicinity of the rated current and a feedback control system for constant voltage control used in the very weak optical output, the circuit configuration disadvantageously becomes complicated and expensive.